1. Field of the Invention
This invention relates to a process for presteaming a zeolite catalyst so as to substantially retain its initial activity and to a process for the preparation of superior zeolite catalysts to be used in acid-catalyzed reactions.
2. Description of the Relevant Art
Acid-catalyzed reactions, e.g., xylene isomerization, toluene disproportionation, etc., result in the rapid degeneration of catalyst activity. It is well known to the art that mild-to-severely steamed zeolite catalysts provide improved stability but suffer from lowered activity in acid-catalyzed reactions.
Much of the prior art in this area deals with severely steamed zeolite catalysts in reactions such as xylene isomerization.
U.S. Pat. No. 4,224,141 discloses a xylene isomerization process with a catalyst steamed at a temperature in excess of 538.degree. C. (1000.degree. F.) for a period of time longer than 15 hours. The resulting catalyst is highly stable, but suffers from lowered activity.
U.S. Pat. No. 4,188,282 discloses a xylene isomerization process using a catalyst with a silica/alumina ratio of at least 200. The catalyst is severely steamed to a lowered activity as described in U.S. Pat. No. 4,016,218 and U.S. Pat. No. 3,965,209.
U.S. Pat. No. 4,236,996 discloses a xylene isomerization process wherein the catalyst is steamed at a high temperature to reduce the activity such that the conversion reaction temperature must be increased by at least 10.degree. C. (50.degree. F.) to equal the conversion capability of an unsteamed zeolite.
U.S. Pat. No. 3,965,209 discloses a process whereby the zeolite is steamed to reduce the alpha activity to less than 500 by treating the zeolite in a steam atmosphere at a temperature of from 250.degree. C. to about 1000.degree. C. (526.degree. F. to about 2026.degree. F.) for from about 1/2 hour to 100 hours.
U.S. Pat. No. 4,326,994 discloses a process whereby a zeolite is steamed to activate or increase its alpha activity.
Other important acid catalyzed reactions include various cracking reactions, such as fluidized catalytic cracking, or hydrocracking. Of special interest in hydrocracking is catalytic hydrodewaxing of distillates and lubricant stocks.
Fluidized catalytic cracking is a well-known commercial refining process. More details of the fluidized catalytic cracking process are disclosed in U.S. Pat. No. 4,309,279, U.S. Pat. No. 4,309,280 and U.S. Pat. No. 3,758,403, the entire contents of which patents are incorporated herein by reference.
Catalytic hydrocracking, conducted in the presence of relatively high hydrogen partial pressures, with particular emphasis on dewaxing of oils by shape selective cracking is disclosed in U.S. Pat. No. Re 28,398, U.S. Pat. No. 3,956,102, U.S. Pat. No. 3,894,938 and U.S. Pat. No. 4,332,670, the entire contents of which patents are incorporated herein by reference.
As discussed in greater detail in these patents, shape selective cracking of either lubricant stocks or of distillates can be conducted using shape selective catalysts, i.e., those having a constraint index of about 1 to 12.